1. Field of the Invention
The present invention relates to multipath detection methods in global positioning system (GPS) receivers used for detecting the positions of movable bodies, such as vehicles.
2. Description of the Related Art
Receivers, such as GPS receivers, for receiving signals that are spectrum-spread by a direct spread system receive signals from a plurality of earth-orbiting GPS satellites to detect the positions of vehicles, vessels, and the like. GPS receivers using three or more GPS satellites are capable of detecting two-dimensional positions (latitude and longitude). GPS receivers using four or more GPS satellites are capable of detecting three-dimensional positions (longitude, latitude, and altitude).
In-vehicle navigation apparatuses use signals received from the GPS receiver to detect the present position of a vehicle, display a map around the present position of the vehicle on a display, and provide route guidance from the present position of the vehicle to a destination.
A GPS receiver in an in-vehicle navigation apparatus includes an antenna 200 that has a directivity in the zenith direction and that is mounted on a vehicle, for example, as shown in FIG. 9. The GPS receiver receives radio waves from three or four GPS satellites 210 via the antenna 200, performs de-spreading and demodulation on the received signals, and regenerates original transmission signals.
Also, for example, Japanese Unexamined Patent Application Publication No. 5-302972 discloses a positioning apparatus in which antennas A and B having narrow grazing angles of θ1 and θ2, respectively, are installed in a vehicle so that diversity reception is performed using the antennas A and B, as shown in FIG. 10. The positioning apparatus switches time-divisionally between the antennas A and B using a switching circuit, so that information from the GPS satellites 210 received by any one of the antennas A and B is used for positioning calculation. Accordingly, time for position detection can be reduced.
However, the known in-vehicle navigation apparatuses described above have problems. For example, when a vehicle provided with such a navigation apparatus travels on a road surrounded by high-rise buildings or the like in an urban area, a GPS receiver may receive multipath indirect waves due to reflection by the buildings. The GPS receiver may not detect the present position of the vehicle accurately under the influence of multipath because an error may occur in position calculation if waves other than pure direct waves from GPS satellites are used for position detection.
In order to solve such a problem, a beam antenna having a directivity of a particular direction may be used to reduce the influence of multipath. It is, however, practically difficult to use an antenna having a directivity of a particular direction for the GPS receiver because the GPS receiver needs to receive radio waves from a plurality of satellites. In contrast, the GPS receiver may be provided with a multipath detection circuit so that multipath noise can be eliminated from reception signals. This arrangement, however, complicates a circuit configuration of the GPS receiver and increases the cost of the GPS receiver.